The removal of sulfur dioxide from industrial flue gases such as that produced by non-ferrous sulfide smelters and fossil fuel-burning power plants, has become a common commercial practice. Effective removal of sulfur dioxide from flue gases, and its conversion into environmentally safe products has become an expensive operation. Reducing the cost of sulfur dioxide removal has the potential of benefiting industry, consumers and the environment.
In conventional sulfur dioxide removal technologies, sulfur dioxide in flue gases may be fixed as one of the following materials, sulfuric acid, liquid sulfur dioxide, elemental sulfur or calcium sulfite/sulfate. The strength of the sulfur dioxide source usually determines the ultimate end product. The limited storage capability and costs arising from long-distance shipping are the main disadvantages of converting sulfur dioxide to sulfuric acid. The problem of producing liquid sulfur dioxide is its limited market. Low strength sulfur dioxide gases are usually fixed as calcium sulfite or sulfate by lime/limestone scrubbing techniques. The drawback with scrubbing is that it converts an atmospheric pollution problem into a solid waste disposal problem.
Since elemental sulfur is easy to ship and store, and is the raw material for many sulfur consuming processes, it is the most desirable product for those sulfur dioxide producers who are located far form their customers. However, existing technologies for producing elemental sulfur are based on sodium compounds and recycling to produce H.sub.2 S (an intermediate to elemental sulfur) that involve many steps, and consequently are very capital intensive and costly to operate.
For example, Engineered Systems International (ESI) has developed a carbonate based process to produce hydrogen sulfide. Hydrogen sulfide produced from the ESI process may be readily converted to elemental sulfur by the Claus reaction. In the ESI regenerable flue gas desulfurization process, the overall reactions are:
Absorption: EQU SO.sub.2 +SO.sub.3.sup.2- +H.sub.2 O.fwdarw.2HSO.sub.3.sup.- PA1 Bisulfite Neutralization: EQU 2 HSO.sub.3.sup.- +CO.sub.3.sup.2- .fwdarw.2 SO.sub.3.sup.2- +CO.sub.2 +H.sub.2 O PA1 Reduction of SO.sub.3.sup.2- : EQU 2 SO.sub.3.sup.2- +3 C.fwdarw.2S.sup.2- +3 CO.sub.2 PA1 Generation of H.sub.2 S: EQU S.sup.2- +CO.sub.2 +H.sub.2 O.fwdarw.H.sub.2 S+CO.sub.3.sup.2- PA1 Claus Reaction to Produce Elemental Sulfur: EQU 2 H.sub.2 S+SO.sub.2 .fwdarw.3S.degree.+2 H.sub.2 O (Claus Plant)
Although the chemicals are regenerable in this process, the process requires a large number of processing units and hence significant capital investment. There are many drawbacks in the existing sulfur dioxide removal processes aiming at hydrogen sulfide production. They arise mostly from the use of carbon dioxide and sodium carbonate. Since carbon dioxide and the alkali carbonates are costly external reagents, recovery of these chemicals is vital to the operation. Recovery of carbon dioxide and alkali carbonates introduces many additional intermediate steps and requires additional processing units. The intermediate steps that are the result of the use of carbon dioxide in the ESI processes are: neutralization of NaHSO.sub.3 by Na.sub.2 CO.sub.3 and collection of CO.sub.2 ; carbon dioxide absorption in two steps (pre-carbonation and pressure carbonation); filtration of bicarbonate solids; and decomposition of NaHCO.sub.3 to Na.sub.2 CO.sub.3 and CO.sub.2.
Outokumpu discloses another method of recovering elemental sulfur from sulfur dioxide gas in U.S. Pat. No. 4,937,057. In the Outokumpu process, sulfur dioxide gas is absorbed into a sodium sulfide or potassium sulfide solution maintained at a pH between 2.5 and 3.5 or a maintained oxidation-reduction potential between -70 and -150 mV in a first reactor and a pH between 3 and 5 or an oxidation-reduction potential between -100 mV and -260 mV in a second reactor. The ratio of sulfur dioxide to sodium or potassium sulfide in the first reactor is maintained between 1.8 and 2.2. The solution from the first reactor is then heated under pressure and temperature to yield elemental sulfur and sodium or potassium sulfate. The sodium or potassium sulfate solution is regenerated into sodium or potassium sulfide for use in sur dioxide absorption.
It is an object of this invention to provide an improved cost effective method of converting sulfur dioxide gas into elemental sulfur.
It is a further object of this invention to provide a carbon dioxide free method of forming elemental sulfur from sulfur dioxide.
It is a further object of this invention to provide a simplified method for forming elemental sulfur from dioxide-containing gases.